What do You mean by 'highlighting'? GLSL syntax highlighting, or highlighting funtion names and parameters, GL constants, etc?
I used both vim (gvim) and emacs, both of them have GLSL syntax highlighting scripts (built-in or available for download,
I never remember, just setup them once and forget about their existence),
You can code your own scripts, it's fairly simple. In fact You can code pretty much anything INSIDE emacs (elisp).
Now I'm sticking with gvim (for me it's faster and simpler, but that is my personal preference).
Beware! Both editors can be hard at the begining, but once You are spoiled by vim/emacs You will have hard time using any IDE

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For me good text editor with built in interpreted language and vcs support beats any IDE ever invented.